It is known from the Applicant's earlier application WO 2011/003789 to provide a pump unit comprising an axial inlet valve. A spring-biased inlet valve member is provided for controlling the supply of fuel to a pumping chamber from a low pressure supply line. The inlet valve member is displaced to an open or closed position in response to a positive or negative pressure differential. The pump unit allows a metered volume of fuel to be delivered to a high pressure manifold. However, the pump unit cannot readily vary the volume of fuel delivered during each pump cycle and additional metering systems may be required for some applications.
The present invention, at least in certain embodiments, sets out to provide an improved pump unit.